Recently, a silica or silica-based thin film that is doped with rare earth atoms has widely been used to manufacture a waveguide amplifier. However, it is essentially similar to a silica or silica-based optical fiber that is doped with rare earth atoms. Furthermore, because the excitation cross section of the rare earth atoms is very low, on the level of 4×10−21 cm2, a high-priced laser for exciting the rare earth element and a WDM structure for dividing a signal light and a pump light are required. Such a waveguide amplifier is no different from an optical amplifier based on an optical fibers. In particular, it is does not offer significant advantages in terms of cost, performance and potential for future development over optical amplifiers based on optical fibers.
Meanwhile, in case of doping rare earth atoms into a semiconductor thin film, the excitation efficiency of the rare earth atoms is greatly increased. This is because carrier recombination gives rise to excitation of the rare earth element in the semiconductor thin film. This excitation mechanism can be schematically expressed in FIG. 1.
FIG. 1 is a schematic view showing excitation and back-excitation of a rare earth element through electron-hole combination. In FIG. 1, a solid line arrow represents an excitation process and a dotted line arrow represents a back-excitation process.
Referring to FIG. 1, when a rare earth-doped semiconductor material is exposed to light, carriers are generated. The carriers are captured into a trap state of the rare earth atom and form electron-hole pairs. Energy generated by recombination of the electron-hole pairs gives rise to excitation of the rare earth atoms through Auger excitation. In this case, there is no particular limitation to the light source for generating carriers as long as the light from the light source is absorbed in the semiconductor. Therefore, there is no need to use a high-priced laser, which has been conventionally used for exciting a rare earth element. In addition, an effective excitation cross section of such an excitation process is 3×10−15 cm2, which is approximately one million times higher than an optical absorption cross section of 8×10−21 cm2. Therefore, more efficient excitation is expected.
However, because all physical phenomena are reversible, back-excitation for reversing the aforementioned excitation can also occur. That is, the “backtransfer” process, “impurity-Auger” process and “exciton-Auger” processes may occur. The excited rare earth atoms form electron-hole pairs instead of emitting light through the back-transfer process, the excited rare earth atoms excite the generated carriers instead of emitting light through the impurity-Auger process, and the generated electron-hole pairs excite the generated carriers instead of exciting the rare earth elements through the exciton-Auger process. For this reason, a conventional rare earth-doped semiconductor has the very low luminous efficiency.
In summary, the silica-based thin film has the high luminous efficiency but the very low excitation efficiency, while the semiconductor thin film has the high excitation efficiency but the very low luminous efficiency. In order to overcome these problems, a method for co-doping silicon nanoclusters and rare earth atoms in a silica/silica-based thin film has been suggested and studied for several years. In this case, the rare earth element provides the high luminous efficiency in the silica thin film. At the same time, because the rare earth element is separated from the silicon nanoclusters by a distance of only several nm, electron-hole combinations formed in the silicon nanoclusters provide the high excitation efficiency. This model is schematically represented in FIG. 2. In FIG. 2, it can be seen that silicon nanoclusters 210 and rare earth atoms 220 are distributed in the silica-based thin film 200 in a state wherein the thin film are co-doped with the silicon nanoclusters and rare earth atoms. However, this model is only conceptual and has not been practically applied. In order for this model to be practically applied, the size and concentration of the silicon nanocluster and the concentration of the rare earth element, must be specifically optimized.